Propylene polymers have been widely prepared on industrial scale because they are excellent in various properties such as mechanical properties, heat resistance, solvent resistance, oil resistance and chemical resistance. The propylene polymers having such properties are broadly used as materials for industrial parts, e.g., automotive parts and electrical apparatus parts, and daily necessaries. The propylene polymers are chemically inert and high in the safety because they have no polar groups in their molecules. However, because of having no polar groups, they are poor in the adhesion properties to other resins. Therefore, for coating the molded product formed from the propylene polymer with, for example, an urethane resin coating, it is required to perform such a treatment to improve affinity of the surface of the product for the resin coating, as electrical treatment (e.g., corona discharge), mechanical surface-roughening treatment, flame treatment, oxygen treatment or ozone treatment. Prior to practicing these surface treatments, generally adopted are a method of beforehand cleaning the surface of the molded product with a solvent such as alcohol or aromatic hydrocarbon and a method of beforehand cleaning it with vapors of a solvent such as trichloroethylene, perchloroethylene, pentachloroethylene or toluene.
In order to conduct any of the above-mentioned surface treatments, an apparatus for such treatment is necessary, and this is a disadvantage in economy. In addition, any of these treatments requires a long period of time. Other than the method of the pretreatment of the molded product, also utilized is a method of coating the molded product with a primer which is able to adhere to propylene polymers and then applying a coating such as an urethane resin coating to the surface of the primer layer. In this method, however, two steps comprising the primer coating (undercoating) and the topcoating (final coating) are necessary, and hence this method also needs a long period of time for the coating. Further, there is involved such a problem that formation of the two-layer structure consisting of the primer layer and the topcoating layer on the molded product needs cost.
With regard to the coating of molded products formed from the propylene polymers, the molded products are generally subjected to some treatment or other as described above. In other words, the molded products formed from the conventionally known propylene polymers must be subjected to a pretreatment prior to the coating.
Accordingly, the advent of propylene polymers which do not need such pretreatments as described above and are excellent in adhesion of coatings has been desired for a long time.